Danazol, a synthetic testosterone derivative, is being administered to patients with congenital coagulation factor deficiencies to evaluate its effect on the levels of the deficient coagulation factor and its possible clinical efficacy. Danazol had been shown to be effective in raising the factor VIII level in patients with mild and moderate hemophilia A. The response in the patients with severe hemophilia A is variable. We are presently involved in a double-blind study of the effects of Danazol in hemophilia A.